1. Field of the Invention
The present invention relates in general to a method for preparation of polyester films, and more particularly to a method for preparing biaxially stretched polyester films with high levelness and excellent release and slip properties by applying thereon a surface coating layer.
2. Description of the Prior Art
As is well known to those skilled in the art, known polyester films, particularly biaxially stretched polyester films, have excellent intrinsic properties, such as optical, mechanical and electrical properties, dimensional stability, heat resistance and chemical resistance. Thus, they have recently been widely used for graphic art films, photo films, magnetic tape films, electrical insulation films, packing films, etc..
However, there occur several problems in processes, such as a printing process, a laminating process, a coating process and the like, for preparation of the several types of polyester film products using the known polyester film. For example, scratches may be easily generated on the film surface due to rolls with which the polyester film comes into close contact during the processes. Furthermore in this case, the higher the processing velocity increases the more scratches are generated. In addition, the hot and humid atmosphere of the processes for preparing the film product detrimentally affects the slip property of the polyester film and this introduces several problems in working performances, for example, run ability of the polyester film and wind ability of the film product, of the polyester film.
On the other hand, when the known polyester film is used for a release material in a fiberglass reinforced plastics (FRP) molding process, the release property of the release material is reduced at the interface between the FRP resin and the release material. Furthermore, the FRP curing conditions influence the release effect of the release material. Therefore, the release material may partially adhere to the FRP resin and, moreover, the sheet molding resin may adhere to the release material as it is heated.
As a result, the known polyester films used for the release films in the FRP forming process have a problem in that they, particularly when they are very thin, may easily tear and/or be delaminated during the mechanical taking-up process thereof, thus causing the forming product to be damaged and, in this respect, to be inevitably reduced in its quality.
Furthermore, this problem of the known polyester film particularly happens when one polyester film is repeatedly used several times. Thus, it is desired to provide a polyester film which is capable of maintaining its own slip and release properties irrespective of repeated use.
To impart excellent slip and release properties to the polyester film, it is preferred to apply at least one surface of the polyester film, a silicone resin as is well known to the art.
Conventionally, the silicone resin is applied to the polyester film surface and is then thermally cured thereon; in the presence of a curing agent. However, this silicone resin intrinsically has an excellent release property so that it has deficient adhesion with respect to the base film. Additionally, if the silicone resin coating layer is strongly rubbed, it may be partially delaminated so as to make the coated product cloudy and, moreover, it may be relatively easily scaled off. Here, the tackiness of the silicone resin layer, with respect to the base film, is gradually reduced as time goes by and the reduction of the tackiness of the silicone resin layer is accelerated when the interface between the silicone resin layer and the base film surface is exposed to the atmosphere.